In recent years, development of clean energy has been desired due to the problem of exhaustion of energy resources, global environmental problems such as an increase in CO2 in the atmosphere, and the like, and photovoltaic power generation using solar cells in particular among semiconductor devices has been developed, put to practical use, and is progressing as a new energy source.
As a solar cell, a bifacial electrode type solar cell has been conventionally a mainstream, in which a p-n junction is formed for example by diffusing an impurity having a conductivity type opposite to that of a single crystal or polycrystalline silicon substrate, into a light receiving surface of the silicon substrate, and electrodes are respectively formed on the light receiving surface and a back surface opposite to the light receiving surface of the silicon substrate. Further, in the bifacial electrode type solar cell, it is also common to achieve higher output using a back surface field effect, by diffusing a high concentration of an impurity having the same conductivity type as that of the silicon substrate into the back surface of the silicon substrate.
In addition, research and development have also been made for a back electrode type solar cell in which no electrode is formed on a light receiving surface of a silicon substrate and electrodes are formed only on a back surface of the silicon substrate (see, for example, PTD 1 (Japanese Patent Laying-Open No. 2006-156646) and the like).
Hereinafter, one example of a method for manufacturing a conventional back electrode type solar cell will be described with reference to the schematic cross sectional views of FIGS. 30(a) to 30(i).
First, as shown in FIG. 30(a), an n-type doping paste 103 is applied on a back surface of a silicon substrate 101 having an n-type or p-type conductivity type, and dried. N-type doping paste 103 is pattern-applied to follow the desired shape of an n-type dopant diffusion region.
Here, as silicon substrate 101, for example, a silicon substrate obtained by slicing a silicon ingot can be used. Further, as silicon substrate 101, it is desirable to use a silicon substrate from which a slice damage layer caused by slicing has been removed. It is noted that the slice damage layer can be removed, for example, by etching with a mixed acid of an aqueous solution of hydrogen fluoride and nitric acid.
It is noted that the surface having n-type doping paste 103 applied thereon is described here as the back surface of silicon substrate 101, and the other surface of silicon substrate 101 serves as a light receiving surface of the solar cell. Hereinafter, the light receiving surface may be referred to as a front surface.
Next, as shown in FIG. 30(b), an n-type dopant is diffused from n-type doping paste 103 into semiconductor substrate 101 to form an n-type dopant diffusion region 113. Thereafter, a residue of n-type doping paste 103 on the back surface of silicon substrate 101 is removed with an aqueous solution of hydrogen fluoride.
Next, as shown in FIG. 30(c), a p-type doping paste 104 is pattern-applied on the back surface of silicon substrate 101 to follow the desired shape of a p-type dopant diffusion region, and dried.
Next, as shown in FIG. 30(d), a p-type dopant is diffused from p-type doping paste 104 into silicon substrate 101 to form a p-type dopant diffusion region 114, and a residue of p-type doping paste 104 is removed with an aqueous solution of hydrogen fluoride.
Next, as shown in FIG. 30(e), a silicon oxide film 105 is formed on the back surface of silicon substrate 101 using a CVD method. On this occasion, a silicon nitride film, or a laminated film of a silicon oxide film and a silicon nitride film may be used instead of silicon oxide film 105.
Next, as shown in FIG. 30(f), a texture structure 110 is formed in the front surface of silicon substrate 101, using for example a mixed acid of an aqueous solution of hydrogen fluoride and nitric acid, or the like. It is noted that, on this occasion, silicon oxide film 105 on the back surface of silicon substrate 101 serves as a protective mask when texture structure 110 is formed, and also serves as a passivation film on the back surface of silicon substrate 101.
Next, as shown in FIG. 30(g), a light receiving surface passivation film 106 is formed on the front surface of silicon substrate 101 using the CVD method. As light receiving surface passivation film 106, a silicon oxide film, a silicon nitride film, or a laminated film of a silicon oxide film and a silicon nitride film may be used. Further, light receiving surface passivation film 106 is a film also serving as a so-called antireflection film.
Next, as shown in FIG. 30(h), portions of silicon oxide film 105 are removed to form contact holes 123, 124 which expose portions of the diffusion regions. To form the contact holes, for example, a known etching paste can be used.
Next, as shown in FIG. 30(i), an electrode for n type 133 electrically connected to n-type dopant diffusion region 113 through contact hole 123 is formed, and an electrode for p type 134 electrically connected to p-type dopant diffusion region 114 through contact hole 124 is formed.
Electrode for n type 133 and electrode for p type 134 can be formed, for example, by printing a known metal paste by a screen printing method and firing the metal paste.